While performing a new Radio Link (RL) addition to a UE's Active Set (AS) following a Wideband Code Division Multiple Access (WCDMA) Event 1a or 1c, the newly added RL might not achieve complete synchronization in the uplink (UL) or downlink (DL) or both, although an RRC Active Set Update Complete message may be received by the Radio Network Controller (RNC) through the other synchronized RLs in the Active Set.
In the existing methodology of RL addition, when a UE requests the addition of a new RL by sending a Radio Resource Control RRC Measurement Report, the RNC evaluates a handover proposal and if successful, sends a NodeB Application Protocol (NBAP) Radio Link Setup Request to the target NodeB to configure the particular RL. Once the RNC receives an NBAP Radio Link Setup Response message, the RNC transmits the RRC Active Set Update message and waits for the RRC Active Set Update Complete message from all the pre-existing RLs in the Active Set. However, the RNC does not wait for an NBAP Radio Link Restore Indication to arrive indicating whether the newly added RL has achieved synchronization. As a result, the newly added RL may remain in the Active Set without achieving synchronization unless it becomes the Serving Cell and a Radio Connection Supervision (RCS) mechanism is triggered for this particular RL.
In the current implementation, it has often been observed that after a new RL has been added to a UE's Active Set either as a result of event 1a or 1c, the UE sends an RRC Active Set Update Complete message without achieving UL synchronization for the newly added RL. This can be established from the fact that no NBAP Radio Link Restore Indication message was received from the Target NodeB for the newly added RL. The RRC Active Set Update message corresponding to this new RL is transmitted by all the RLs and the RRC Active Set Update Complete message is received by the RNC from all the old RLs in the Active Set except the newly added one (which has not achieved synchronization).
Subsequently, an addition/replacement process for an additional new RL may be initiated. An RRC Active Set Update (Addition) message is transmitted to add the new RL, and synchronization for the newly added RL is also confirmed upon reception by the RNC of an NBAP Radio Link Restore Indication message. However, the RL which was added prior to this new RL addition procedure continues to remain unsynchronized. This can be confirmed by checking the RLS_TAG corresponding to this RL in the Internal RC Supervision message. As a result, the RRC Active Set Update Complete message is not received by all the RLs present in the current Active Set. Consequently, the RNC does not consider the Active Set Update procedure to be completed and waits for the same to arrive via all existing RLs until an Active Set Update Timer expires.
After expiry of the Active Set Update Timer (which may range for example, from 5-15 seconds), the UE's call is released. This occurs irrespective of the fact that the serving RL had adequate signal strength to sustain the call. Any RRC Measurement Report sent by the UE in the meantime will be buffered during this duration (wait period for RRC Active Set Update Complete message) and will not be evaluated. As a result, any valid candidate cell that was reported for addition/replacement could not be evaluated thereby missing the opportunity to add a stronger cell to the Active Set. Thus, the unsynchronized RL consumes physical resources such as channel element, codes, and DL power without providing any gain to the call.
This issue has severe impact on High-Speed Downlink Packet Access (HSDPA) calls as well. A new RL is added to the Active Set, but does not become the serving High-Speed Downlink Shared Channel (HS-DSCH) cell initially. The RRC Active Set Update message is transmitted on the downlink, and the RRC Active Set Update Complete message is received. However, the newly added RL could not achieve UL Synchronization, and no NBAP Radio Link Restore Indication message is received by the RNC for this specific RL. At a later stage, when this new RL (still having unsynchronized UL) becomes a candidate for the best cell, event 1d is reported by the UE. Accordingly, this RL becomes the serving HS-DSCH cell and the RCS supervision starts. After expiry of a HsDschRclostT timer, the connection is released.